Electrical steel sheet with axes of easy magnetization of crystals aligned in substantially the same direction in the steel sheet as a whole is called “grain-oriented electrical steel sheet”. The direction of the axes of easy magnetization matches with the rolling direction of the steel sheet. This steel sheet is extremely superior as a material for a transformer core with a constant magnetization direction.
The core loss when magnetizing grain-oriented electrical steel sheet by alternating current is divided into eddy current loss and hysteresis loss. The eddy current loss is further divided into classic eddy current loss and abnormal eddy current loss. Classic eddy current loss is proportional to the thickness of the steel sheet, so is reduced by making the thickness of the material smaller. Abnormal eddy current loss is loss due to the large eddy current locally generated due to movement of the domain walls and becomes smaller proportional to the distance between domain walls of 180° magnetic domains long in the rolling direction. Therefore, various techniques have been devised for increasing the fineness of the magnetic domains to lower the core loss.
By giving periodic stress in a linear form substantially vertical to the rolling direction and in the rolling direction near the surface of the steel sheet, the distance between 180° domain walls becomes narrower.
Therefore, for example, as disclosed in Japanese Patent Publication (B2) No. 6-19112, the method has been devised of scanning and irradiating a steel sheet in the width direction by a focused laser beam to impart stress to the steel sheet and is currently in actual use.
This method of control of magnetic domains by laser is a method of using strain and stress to reduce the core loss. This is not accompanied with changes in the shape of the electrical steel sheet as with formation of melted parts or grooves, so if performing stress relief annealing, the stress imparting effect disappears. However, when not performing stress relief annealing, a 10% or higher effect of improvement of the core loss is easily obtained, so the electrical steel sheet produced by this method is in extremely high demand as a material for a stacked transformer not including annealing in the production process.
On the other hand, hysteresis loss is loss due to the magnetization curve, that is, the hysteresis curve, that is, the core loss component sensitive to the stress of the steel sheet. Therefore, impartation of stress by laser irradiation has the problem that it leads to an increase in the hysteresis loss.
Therefore, to suppress the unnecessary stress to the maximum, it can be said to be desirable to introduce effective stress in as narrow a region as possible. For example, in the art disclosed in WO2004/083465, by making the size of the focused spot on the electrical steel sheet in the rolling direction not more than 0.2 mm, stress is imparted to a narrow region and superior properties are obtained.
However if making the focused size smaller while leaving it circular, the power density of the focused spot increases, but the time during which the focused spot passes over any point on the steel sheet, that is, the beam residence time of the beam size divided by the scan speed, becomes shorter. The beam passes before almost any transfer of heat occurs in the thickness direction of the steel sheet.
In such a case, heat excessively enters just the extreme surface layer of the steel sheet. In some cases, the surface layer melts. However, almost no heat is transferred in the depth direction of the sheet thickness, so the heat elevated region is limited to near the surface, so there is the problem that it not possible to sufficiently introduce into the steel sheet the stress required for making the magnetic domains finer.
Therefore, as disclosed in WO2004/083465, the method of making the shape of the focused spot an ellipse long in the scan direction of the beam may be considered. In this method, while leaving the stress width of the rolling direction narrow, the power density can be suppressed and the beam residence time on the irradiated point can be increased, sufficient stress can be given in the steel sheet thickness direction, and an excellent core loss property can be obtained.
However, if using the technique of making the shape of the focused spot an ellipse long in the scan direction of the beam in a production facility where the steel sheet moves in the rolling direction at a high speed, the following problem arises.
FIG. 3 gives schematic views showing, in the technique of focusing a laser beam on a steel sheet into a focused spot LS with a short rolling direction size dl and a long width direction size dc (hereinafter referred to as an “elliptical spot”) and making that spot scan the steel sheet in the direction C approximately vertical to the rolling direction L at a speed Vc so as irradiate the steel sheet with a laser beam, the position of the elliptical spot LS and the laser beam-induced damage TC left on the steel sheet in the case of aligning the long axis direction of the elliptical spot with the scan direction C and the steel sheet moving in the rolling direction L by the speed V1.
In this case, if assuming that the front end of the beam passes through a point A on the steel sheet at the time t=0, then when the tail end of the beam passes near the point A at the time t=ts, the point A has already moved by exactly H in the rolling direction due to movement of the steel sheet and the tail end of the beam passes the point A′.
Therefore, the laser beam-induced damage TC on the steel sheet, that is, the path along which the steel sheet actually receives the laser beam, as shown in FIG. 3, is slanted by an angle θ proportional to the ratio of Vl and Vc and has a width proportional to H.
That is, even if making the short axis dl of the elliptical beam extremely small to lower the core loss, when the steel sheet is moving at a high speed, the effective laser irradiated width actually received by the steel sheet has a width proportional to H and the effective laser irradiated width cannot be made narrower. Further, H is expressed by Vl×dc/Vc, so the effect of the speed of movement of the steel sheet becomes larger when the rolling direction line speed Vl of the steel sheet is large, that is, when the production speed is slow.
Further, if aiming at better improvement of the magnetic properties and giving the elliptical beam a finer short axis, it is necessary to make the long axis longer so as to keep down the power density. As a result, the beam residence time becomes longer, so the effect of the movement of the steel sheet on the effective laser irradiated width becomes greater. That is, there was the problem that the effect becomes greater the better the magnetic properties and the higher the production capability in the production facilities.